Field of the Invention
The present invention relates to switchable macroscopic quantum state devices and to methods of operation and uses of such devices. Such devices have particular, but not necessarily exclusive, application in the area of information storage, such as memory cells.
Related Art
The size scaling of Si-based integrated-circuit components (CPU and memory), has followed the well-known Moore's law for the last few decades. However, this doubling of capacity every two years, due to a reduction in feature size, is predicted to come to an end for the case of non-volatile (flash) memory: the thickness of the oxide insulator layer in such field-effect transistor devices will be insufficient to prevent electrons trapped at the gate from tunnelling away, thereby causing volatility. There is a need, therefore, for a new scalable, non-volatile memory technology to replace flash.
Phase-change (PC) materials are of interest for non-volatile electronic-memory technology to replace flash memory. Suitable phase change materials have amorphous and crystalline states with different values of electrical resistivity, thereby enabling the encoding of bits of information. Furthermore, suitable phase change materials may allow fast, reversible transformations between the amorphous and crystalline states by suitable control of the temperature and heating/cooling rate of the material. Typically, heat is applied to the material to control the temperature by passing electrical current through the material and relying on Joule heating. Thus, the ability of phase-change (PC) materials to be reversibly and rapidly transformed between high-resistance amorphous and low-resistance crystalline states by joule heating via imposed current pulses provides the potential for recording binary bits of information.
Most PC memory materials investigated to date have been Ge—Sb—Te (GST) compounds, of which Ge2Sb2Te5 (GST 225) is probably the most studied. A useful review of PC memory technologies is set out in Wong et al (2010).
PC memory is one example of resistance-based memory. Other examples exploit different mechanisms for developing different electrical resistance in the memory cell.
In US 2011/0227031, a two-terminal memristor device is disclosed with an active region containing a primary material for controlling the flow of charge between the terminals and a secondary material. The composition of the primary material is ABO3, where A is a divalent element and B is Ti, Zr or Hf. The secondary material is provided in order to store oxygen ions generated during the formation of the active region, in order to prolong the useful life of the device. US 2011/0266513 discloses a memristor device which similarly has an active region with primary and secondary sub-regions, control of the resistance of the active region being by control of dopants in the primary sub-region from the secondary sub-region.
US 2011/0309321 discloses a memristor with a switching layer held between first and second electrodes. The switching layer is a composite of an insulating phase and a conductive phase, which can be treated to provide a conductive channel between the electrodes.
US 2012/0026776 discloses a memristor in which the active layer is subject to oxygen ion reconfiguration under the influence of an applied electric field to provide control of the electrical resistance of the active layer.
US 2012/0113706 discloses a memristor based on a mixed metal valence phase in contact with a fully oxidized phase. The mixed metal valence phase in effect is a condensed phase of dopants for the oxidized phase, the dopants drifting into and out of the oxidized phase in response to an electric field, thereby affecting the resistance of the device.